I wouldn't put much faith in Killzone, I heard the ending of 3 is like one of the worst of the generation (never played it).
I'm actually playing through 3 right now. Story is entertaining but pretty bad and disjointed, like Killzone 2. I have also heard Killzone 3 ends pretty badly, but Shadowfall takes place like 20 or 30 years later and seems to take advantage of the lore/setting more than usual. I'm hoping that Guerilla learns from their mistakes. They certainly did when it came to the shooting mechanics, which were awful to me in KZ2, but massively improved in KZ3.
Will it replace Crysis 3 has the PC benchmark?
Seems to be more optimized than Crysis 3 was on release, so I guess we'll see.